Liquid contents gauges



March 13, 1956 E. B. Moss LIQUID CONTENTS GAUGE-S Filed Aug. 23, 1950 United States Patent O LIQUID CONTENTS GAUGES Eric Eeecroft Moss, Cricklewood, London, England, as-

signor to S. Smith & Sons (England) Limited, London, England Application August 23, 1950, Serial No. 181,089 Claims. (Cl. 73-304) This invention relates to devices for indicating the quantity of a liquid Within a container which depend for their operation upon the variation of the capacitance of a condenser 'in accordance with the dielectric constant of the material 'between its plates. Such devices are known in which a plurality of condensers connected in parallel `are distributed within the space which may contain liquid. Those condensers (or parts of condensers) immersed in the liquid will have their capacitances increased by an amount (K-l times that when not immersed. (K lbeing the dielectric constant of the liquid) and thus if the condensers are distributed substantially uniformly throughout the volume of liquid their capacitance will give a measure of a function of the dielectric constant and volume of the liquid. However, liquids vary in dielectric constant and density and in many circumstances the quantity of liquid is required not in terms of volume, but of mass. In such circumstances an indication is required which is compensated both for the density and dielectric constant of the liquid, and is thus also compensated for its temperature.

Devices of the kind referred to may be used as fuel contents gauges for aircraft, the type of fuel gauge having a oat operating some kind of telemetric device being undesirable for a variety of reasons.

The object of the present invention is to provide a device of the type referred to indicating the mass of fluid in a container, the said indication being independent of both the dielectric constant and the density of the liquid.

According to the present invention, a device of the type referred to comprises a reference condenser whose capacitance varies linearly with the quotient of the dielectric constant of the liquid minus unity and the density of the liquid, means to generate a rst electric current substantially proportional to capacitance of said reference condenser, means to generate a rst backing'ol current equal to said first current when dielectric constant of said liquid is unity, condenser means distributed through the space to be occupied by said liquid, the capacitance of said condenser means varying substantially linearly with the product of the dielectric constant minus unity and the volume of said liquid in said space, means to generate a second current proportional to capacitance of said condenser means, means to generate a second backing ot current equal `to said second current when vol-ume of liquid in said space is zero, and a ratiometer indicating the ratio of the difference `between said rst and trst backing-off current and said second and second backingoff currents, said ratio being substantially proportional to the mass of liquid in the aforesaid space.

Preferably said rst and second backing off currents are proportional respectively to the capacitances of first and second trimmer condensers.

Preferably said reference condenser comprises uniformly spaced plate means supported by a suitable buoyant member in a vessel containing liquid identical with that in said space,-the amount of the plate means sublCC merged within said liquid being inversely proportional to the density of said liquid.

An embodiment of the invention in which it is 4applied to a fuel contents indicating system will now be described with reference to the accompanying drawings of which Figure l shows a circuit diagram of the whole device and Figure 2 illustrates in further detail the reference condenser 1, and the condenser means 2, and the relation between them.

An A. C. source 6 is connected to the primary of a transformer 5 having two centre-tapped secondary windings 7 and 8. Refererence condenser 1 is connected in series with the iirst preset trimmer condenser 3 across winding 7, while the junction of the condensers 1 and 3 is connected, through a suitable rectifier 25 and one winding 9 of a ratiometer, to the centre tap of winding 7. The condenser means 2 are connected in series with the second preset trimmer condenser 4 across winding 8 and the junction of these condensers is connected through a suitable rectifier 26 and the second winding 10 of the ratiometer to the centre-top of winding 8.

Reference condenser 1 comprises a hollow metallic cylinder 15 and a further metallic cylinder 16, coaxial therewith 'but attached thereto and spaced therefrom by insulators, the cylinders 15 and 16 floating in a quantity 17 of fuel contained in an auxiliary tank 24, and having connecting leads 20. Condenser means 2 comprise a number of pairs of insulated plates 19 distributed about the main fuel tank 18 and connected in parallel to leads 21 so that the capacitance of the portions of the plates immersed in fuel is proportional to the volume of fuel. Fuel is pumped continuously by a pump 22 from the tank 18 to the auxiliary tank 24, while a return pipe 23 is provided between the auxiliary tank 24 and the main tank 18, thereby ensuring that the fuel 17 is Valways representative of the fuel in the tank 18.

The ratiometer is of a wellknown type, coils 9 and 10 being set at right angles and the pointer 13` being controlled by a permanent magnet 11 pivoted about an axis 12 and reading against a scale 14.

The action of the gauge is as follows:

The capacitance of reference condenser 1 is of the form where A and B are constants, p the density and K the dielectric constant of the fluid, as is seen by the application of well-known hydrostatic and electrical principles.

If the effective impedance of transformer winding 7 and the input impedance of the rectifier feeding ratiometer winding 9 is very much less than that of either condenser 1 or condenser 3 at the frequency of source 6, the current through winding 9 will be clearly proportional to the difference between the capacitances of these condensers.

lf condenser 3 is therefore adjusted to have a capacitance A, i. e. a capacitance equal to that of condenser 1 when the fuel 17 has dielectric constant unity (or more conveniently, when the auxiliary tank 24 is empty) the current in coil 9 will be proportional to p The capacitance of condenser 2 will be of the form where V is the volume of liquid in tank 18 and C, D are constants. Condenser 4 is adjusted to have a capacitance equal to that of condenser 2 when tank 18 is empty so, if the mpedances of condensers 2 and 4 satisfy the same conditions with regard to the impedance of winding 8 and coil 10 as those satisfied by condensers 1 and 3, winding 7 and coil 9, the current through winding 10 will be proportional to (K--1).V.

The ratiometer will indicate the ratio of these currents, through windings 9 and lil, i. e. it will indicate, when the scale is suitably calibrated, the, quantity pv, whichv is equal to the mass of fluid.

The` gauge is adjusted as follows:

With tanl; empty and quantity of fuel i7 in tanl; 2li, trimmer 4 is adjusted until magnet 1l is parallel with the axis of coil 9; there being then no current coil lil. Then with a quantity of fuel in the tank iti but with no fuel in tank 24, trimmer 3 is adjusted until magnet ll is parallel with coil lil.

lt will be clear to those skilled in the art that the. process of adiustment described above will also result in the removal of the effects ofv the various stray capacitanccs which will inevitably be present, and., it will also clear that if transformer windings '7 and are not in 'fact centretapped the. gauge will still be operative, as all that the currents through trimmer 3. and 45- are required to do is,

to cancel out the current through what might be called the empty capacitance of condensers l and 2 respectively.

The ratiometer has been described as of the moving magnet type for the sake of simplicity. However, such a ratioineter has the disadvantage that the scale becomes very crowded towards the high end, and it is preferable to use in practice a ratiorneter of the type described in British. patent Specification No. 561,366 which has an equal division adjusted scale with the high values eX- panded to increase readability.

l claim:

l. liquid contents gauge for indicating the quantity in a given space, reference condenser means comprising a vessel, means to supply to said vessel a sample of the liquid in the said space, first and second cylindrical plates spaced apart from each other, means attached to said plates for forming a buoyant assemblage having a cylindrical dotation. portion, the axes of said plates and said flotation portion being substantially parallel to one another, whereby said plates are supported by flotation in said liquid sample, the space between said plates being accessible to said liquid sample, whereby the capacitance between said plates varies substantially linearly with the quotient of the dielectric constant minus unity and the density of the liquid sample.

2. A liquid contents gauge for indicating the mass of liquid in a container comprising the combination of a reference condenser, said reference condenser comprising two plates spaced apart from each other; means attached to said plates for forming therewith a buoyant assemblage adapted to support said plates by flotation in a. vessel adapted to contain liquid identical with that contained in said container, the space between said plates being occupied by liquid to an extent dependent upon the density of the liquid, said plates and said buoyant assemblage being so shaped and arranged that the capacitance of the reference condenser varies linearly with the quotient of the dielectric constant minus unity, and the density of the liquid; a first A. C. source connected in series with said reference condenser to produce a first electric current substantially proportional to its capacitance; means to produce a rst backing-ofi current equal to said irst current when the dielectric constant of said liquid is unity; xneans to combine said first current and said first backingoff current in opposition to give a first metering current proportional to the quotient of the dielectric constant minus unity, and the density ofthe liquid; condenser means distributed through the space to be occupied by said liquid, the capacitance of said condenser means varying substantially linearly with the product of the dielectric constant minus unity, and the volume of said liquid in saidfspace; a second A. C. source connected in series with said. condenser means to produce a second current substantially proportional to its capacitance; means to produce a second backing-off current equal to said second current when the volume of" liquid in said' space is zero; means to combine said second current and said second backing-off current in opposition to give a second metering current proportional to the product of the dielectric constant minus unity, and the volume of the liquid in the said space; a ratiometer; and means to apply said first and second currents to said ratiorneter, said ratiorneter serving to indicate the ratio between said first and second metering currents and thus to give an indication substantially proportional to the mass of liquid in the aforesaid space.

3. A liquid contents gauge for indicating the mass of liquid in a container comprising the combination of a reference condenser, said'referencc condenser comprising two cylindrical plates spaced apart from one another in l coaxial relation; means attached to said plates for forming therewith af buoyant ,assemblage having a cylindrical ilotation portion adapted to support said plates by flotationy in a vessel adapted to contain a sample of the liquid contained in said container, the space between said plates being occupied. by liquid to an extent dependent upon the density of the liquid, said plates and said cylindrical dotation portion having their axes parallel, whereby the capacitance of the reference condenser varies linearly with the quotient of the dielectricv constant minus unity, and the density of the liquid; a first A. C. source connected: in series with said reference condenser to produce a first. electric current substantially proportional to its capacitance; means to produce a iirst backing-off current equal to said first current when the dielectric constant of saidliquid is unity; means to combine said first current and said first backing-olf current in opposition to give a first metering current proportional to the quotient of the dielectric constant minus unity, and the density of the liquid; condenser means distributed through the space to be. occupied by said liquid, the capacitance of said condenser means varying substantially linearly with the product of the dielectric constant minus unity, the volume of said liquid in said space; a second A. C. source connectcd in series with said condenser means to produce a second cu rent substantially proportional to its capacitance.; means to produce a second backing-olf current equal to said second current when the volume of liquid in said space is zero; means to combine said second current and said second backing-olf current in opposition to give a second metering current proportional to the product of the. dielectric constant minus unity, and the volume. of the liquid in the said space; a ratiometer; and means to apply said first and second currents to said ratiometer, said ratiometer serving to indicate the ratio between said irst and second metering currents, and thus to giveA an indication substantially proportional to the mass of liquid. in the aforesaid space.

4. In a device to measure liquid contents, the combination ofy a main compartment, an auxiliary compartment connected to said main compartment, means to maintain the volume of liquid in said auxiliary compartnient substantially constant, means to circulate liquid to be gauged from one compartment to another to assure representativeness of the liquid in said auxiliary compartment, first electric means to measure a value which is a function ofthe density and dielectric constant of the liquid in said auxiliary compartment, a second electric means to measure a value which is a function of the voiume and dielectric constant of the liquid in said main compartment, a computing circuit, both said electric means being connected to said computing circuit as inputs, said computing circuit producing outputs respectively proportional to the product of a term which is the function of the dielectric constant and the reciprocal of the density andito the product of the term and the volume ofthe liquid-in the main compartment, and a rationieter connectedto the outputs of said computing circuit Whereby'to indicate-a resultant-signal which represents the product of the density and the volume of the liquid being gauged.

5. in a liquid contents gauge for indicating the quantity of liquid. in a given space, reference condenser means comprising a Vessel, means to supply to said vessel a sample of the liquid in the said space, first and second plates spaced apat from each other, means attached to said plates for supporting them by ctation in said liquid sample, the space between said plates being accessible to said liquid sample, said means and said plates being so shaped and disposed that the capacitance between said plates varies substantially linearly with the quotient of the dielectric constant minus unity and the density of the liquid sample.

References Cited in the file of this patent UNITED STATES PATENTS 2,3%,502 Freystedt Nov. 3, 1942 2,377,275 Smith iviay 29, 1945 2,541,743 Broekman et al. Feb. 13, 1951 2,581,085 Edelman Ian. l, 1952 FOREGN PATENTS 441,576 Great Britain Jan. 22, 1936 114,980 Sweden Sept. 18, 1945 

